Purpose of the test
The F/A-18 was selected by the swiss army to serve as a multi-role fighter jet. Its design was adapted to meet the requirements specific to Switzerland. As part of this modification program, the structural integrity needed to be checked. Parallel to FEM simulations, fatigue tests were performed on a full-scale physical prototype. The prototype was mounted on a hydraulic test bench, allowing to simulate typical stress situations on the airplane.
In order to validate the correctness of the used model in the FEM simulation, the stiffness of the full-scale test specimen needed to be measured.
Since a full scale F/A-18 has a span of 14 m, and a length of 17 m, multiple optical trackers were needed to cover the left side of the airplane.
A total of 4 optical trackers were used. The position of every optical tracker was determined in advance using a CAD file of the airplane. The ideal positions were then checked against the possibilities of the actual setup.
Using reference points on the airplane, all optical trackers were aligned to the common coordinate reference system of the CAD file of the airplane. This was necessary to combine the measurements of the individual cameras. Additional advantages of aligning the optical trackers to the CAD coordinate system were that the results could be evaluated during the measurements, and that the IR (Infra-Red) markers could be placed in pre-determined locations on the airplane by setting the optical tracker in measurement mode an looking at the 3D values.
In total 96 IR markers were used to total 288 measurement channels. This data was fed online during the test to a central data-acquisition system, so that load-displacement graphs could be monitored live. This has the advantage of increased control over the experiment. No post-processing is needed, the validity of the measurements can be checked immidiately; this in contrast with string potention-meters or LVDT's, that need extensive preparation and post-processing.
The main advantages of the optical trackers over instrumenting with LVDT's or potentiometers are:
- Short set-up time compared to LVDT's/potentiometers. With the optical trackers the set-up, measurements and wrap-up were performed in 3 days.
- Using the IR markers, more measurement points could be instrumented
- The data was directly available in the CAD coordinate system, making a quick comparison with FEM simulation possible
- The data was available for monitoring and controlling purposes during the experiment